Barometric altimeter

ABSTRACT

Apparatus is disclosed for comparing the magnitude of an exponentially changing signal with time respectively with that of a signal proportional to the atmospheric pressure at a reference altitude and that of a signal proportional to the atmospheric pressure at the altitude to be measured; further, their respective times of coincidence are measured. The altitude can then be measured because this time difference is proportional to the difference between the altitude to be measured and the reference altitude and, furthermore, the accurate altitude can be measured executing a correction of the temperature distribution.

a t Muted @tates Patent 1 i 1 Shimomui'a 1 1 May 1, 1973 1 BARUMETRHCALTIMETER Primary Examiner-Donald O. Woodie] [76] Inventor: NaonobuShimomura, No. 13-8 Attorneyfiflarry John Staas et Sakuragaoka-cho,Shibuya-ku, Tokyo, Japan [57] ABSTRACT [22] Filed May 12 1972 Apparatusis disclosed for comparing the magnitude of an exponentially changingsignal with time respective- [21 Appl. No.: 254,784 1y with that of asignal proportional to the atmospheric pressure at a reference altitudeand that of a signal proportional to the atmospheric pressure at the al-[301 Foreign Appllcamm Priority Data titude to be measured; further,their respective times May 12, 1971 Japan ..46/31858 of coincidence aremeasured. The altitude can then be measured because this time differenceis proportional [52] US. Cl ..73/384 to the difference between thealtitude to be measured [51] Int. Cl. "G011 7/00 and the referencealtitude and, furthermore, the accu- [58] Field of Search ..73/384, 386,387, rate altitude can be measured executing a correction 73/393;235/183, 150.2, 150.22 of the temperature distribution.

[56] References Cited 18 Claims, 10 Drawing Figures UNITED STATESPATENTS 3,693,405 9/1972 Shimomura ..73/384 ,4 REF. V0 VOLTAGECOMPARATOR to GEN.

COUNTER PRESS VI VOLTAGE COMPARATOR CONV.

EXP. VOLTAGE V2 ZZ 7 GEN.

Patented May 1, 1973- 5 Sheets-Sheet 1 'FIG.

FIG.2

COUNTER COMPARATOR REF.

VOLTAGE GEN.

COMPARATOR CONTROL CKT PRESS- FIG. 3

VOLTAGE couv.

EXP.

VOLTAGE GEN.

TEMPERATURE Patented May 1, 1973 5 Sheets-Sheet 2 FIG. 4

ALTITUDE FROM ILOOO SEA LEVEL (m) FIG. 5

Ho; Po,V

I Hn, Pn, Vn

Patented Ma 1, 1973 5 Sheets-Sheet :6

F e. 6 3e INDICATOR 4 35 8-0 W CONVERTER ARITHMETIC 2| 26 CIRCUIT\KSWITCH y SWITCH MEMORY 5 a J COUNTER k 2. 26ll l0 I VOLTAGE 28 i.CONTROLLED OSCILLATOR 534 28G 24 25 655E Q v33 30 ADDER COMPARATORIACCUMUEQTOR" I .26IIXIO XHn MEMORY 32 J D-A CONVERTER BAROMETRICALTIMETER BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to barometric altimeters, and particularly to thoseaccurate barometric altimeters to be used for aircraft.

2. Description of the Prior Art The principle of a barometric altimeteris based on a fact that the atmospheric pressure decreases as thealtitude increases, and the altitude can be obtained by the measurementof the atmospheric pressure. In the conventional barometric altimeter,the atmospheric pressure is measured from the deformation of an aneroidcapsule due to the change of the atmospheric pressure and thisdeformation is indicated after being mechanically magnified by a trainof gears. Thus, as the deformation is mechanically magnified, even aslight friction in a movable member results a large error. This error isnot proportional to the altitude and is not decreased at low altitude,and hence an aircraft may get into danger in a low altitude flight. Inorder that the meter be graduated in a linear scale, a logarithmictransformation mechanism with the mechanical magnification ofdeformation of the capsule is necessary and the mechanical accuracy ofsuch transformation mechanism is very critical. In those applicationswhere the altimeter is incorporated in an automatic altitude reportingsystem, a complex high-cost encoder of the contactless type such as anoptical encoder has been used.

To overcome these problems, Applicant has proposed barometric altimeterwhich senses the altitude by measuring the time interval between thetimes when an exponentially changing voltage with time, coincidesrespectively with a voltage proportional to the atmospheric pressure ata reference altitude and a voltage proportional to the atmosphericpressure at the altitude to be measured. The relation between theatmospheric pressure P at the reference altitude H and the atmosphericpressure P, at the altitude H, to be measured, is given as follows,

H,H ==k(logP,,-logP,) (I) where, k is a constant determined by the meantemperature. An exponential voltage V is represented as follows,

V Ve" 2 where, V is an initial voltage and c is an inverse of atime-constant. If the times, when voltages V and V, coincide with theexponential voltage V are denoted respectively as 1,, and 1,, thevoltage V,, proportional to the atmospheric pressure P and the voltageV, proportional to the atmospheric pressure P, are represented asfollows,

Ve' V (3) Therefore,

The voltages V and V, are proportional respectively to the atmosphericpressure P,, and P,, and hence,

The altitude H, to be measured can be obtained from the equation (6) ifthe times t and 1,, when the exponential voltage V coincidesrespectively with voltages V,, and V,, are measured.

BRIEF SUMMARY OF THE INVENTION An object ofthis invention is to measurethe accurate altitude executing a correction for the temperature in suchbarometric altimeter as described above.

Another object ofthis invention is to make an altimeter executeautomatically the temperature correction with relation to the altitude.

A further object of this invention is to make the altimeter execute adigital measurement ofthe altitude.

Briefly stated, the barometric altimeter of this invention is composedof means for generating an electric signal which changes exponentiallywith time; means for generating electric signals proportionalrespectively to the atmospheric pressure of the altitude to be measuredand that of a reference altitude; means for counting which is controlledby the times when the electric signals coincide with each other; andmeans for correcting the counted value obtained by the counting meansfor temperature variations.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantagesof the present invention will become more apparent by referring to thefollowing detailed description and accompanying drawings, in which:

FIG. 1 is an explanatory graph of the principles of this invention.

FIG. 2 is a block diagram ofthe barometric altimeter of this invention.

FIG. 3 is a block diagram of an embodiment of the circuit whichgenerates exponentially changing voltages in FIG. 2.

FIG. 4 is a graph showing the ICAO standard atmospheric temperaturedistribution.

FIG. 5 is an explanatory graph of the temperature correction by thebarometric altimeter of this invention.

FIG. 6 is a block diagram of an embodiment of the temperature correctioncircuit in the barometric altimeter of this invention.

FIG. 7 is a characteristic curve explaining the operation of the circuitof FIG. 6.

FIG. 8 and FIG. 9 are block diagrams of the two other embodiments of thetemperature correction cir cuits ofthe barometric altimeter of thisinvention.

FIG. 10 is an embodiment showing a modification of a part of the circuitof FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A detailed descriptionofthis invention is made referring to FIG. 1. Although voltage, current,etc. may be used as an electric signal representing the atmosphericpressure, the description is made hereunder in terms of voltages. V isthat voltage proportional to the atmospheric pressure P at a referencealtitude; V, is a voltage proportional to the atmospheric pressure P, atthe altitude to be measured; and V is an exponentially changing voltageindicated in the equation (2), given above. The altitude to be measuredis obtained from the equation (6), by measuring the times t and I, whenthe value of this voltage V coincides respectively with those ofvoltages V and V FIG. 2 shows a barometric altitude measuring circuitcomposed of a reference voltage generator 1 for generating a firstelectrical signal or voltage V as an electrical signal proportional tothe atmospheric pressure P at a reference altitude H a pressure-voltageconverter 2 for generating a second electrical signal or voltage V as anelectric signal proportional to the atmospheric pressure P at thealtitude H to be measured; and exponentially changing voltage generator3 for generating a third electric signal or voltage V as an electricalsignal changing exponentially with time; a comparator 4 which generatesa pulse at the time 2 when the voltage V coincides with the voltage V acomparator 5 which generates a pulse at the time r when the voltage Vcoincides with the voltage V a counter 6 which counts a frequency of anoscillator during the interval between coincided pulses generated at thetimes t and I from the comparators 4 and 5; and a control circuit 7which controls the above-mentioned oscillator, the exponential voltagegenerator 3, and the correction for the temperature.

According to the Laplaces formula, the relation between the altitude Hto be measured and a reference altitude Hon") is shown by the followingequation, H H l8400(log P log P l+0.003670) 7 where, 6 is the meantemperature (C) of the atmosphere between altitudes H and H The unitmeasurement of altitude is the meter in the equation (7), but the footunit measurement may be used by changing the equations coefficient. Anyunits of the atmospheric pressure P and P may be used.

Now, when 0 l in equation (2), that is, the'timeconstant is 1 second andcommon logarithms are used, the equation (1 is presented as follows, H,H=2.3026 k(log ,,P,,log P,) it

Then, from the equations (7) and (8), the next equation is obtained,

k=799l.03(l +0.003670) 9 Therefore, counter 6 indicates directly thealtitude expressed in meters if the counting frequency of the counter 6is set at the value of equation (9). To obtain the accurate altitudefrom a runway in case of landing of an aircraft, the voltage Vproportional to the atmospheric pressure at the landing point is set onthe basis of the weather information from the landing airfield, and thecoefficient k obtained from the equation (9) by the temperature of thelanding point, is given as a counting frequency of the counter 6. Eventhough there are some errors due to the temperature difference betweenthe mean temperature and the temperature at the landing point when thealtitude is high, these errors decrease according to the approach tolanding point of the aircraft and, as an extremely accurate altitudemeasurement can be obtained at the time of landing, landing failures canbe prevented. The counting frequency of the counter 6 can be adjustedmanually by a dial ofa variable frequency oscillator according to thetemperature. It is preferable to use a frequency synthesizer to performthis function most accurately.

As shown in FIG. 3, an exponential voltage generator 3 may beillustratively comprised of an integrator circuit, a differentiatorcircuit, or a semiconductor switch 12, a capacitor 15, and a resistor16. In FIG. 3, an initial voltage V is applied to a terminal 11 and thecapacitor 15 is charged with this voltage V by closing the switch 12 bya pulse applied to a terminal 13. As a result, an exponential voltage Vof equation (2) is obtained by rendering the switch 12 non-conductive inresponse to a pulse applied to a terminal 14. The pulses applied to theterminals 13 and 14 are obtained from the control circuit 7 shown inFIG. 2. The exponential function generated by the circuit of FIG. 3decays with time, but an exponential function which increases with timecan also be employed for the purpose of this invention. Such increasingexponential waveform is generated, for example, by an integrator withpositive feedback circuit. If we use alternatively such an exponentialfunction instead of a decaying type, the timing of the coincident pulseswhich are generated at the times of crossing over the exponentialwaveform and the two signals representing the reference altitude and thealtitude to be measured are reversed.

In both cases, decaying or increasing, the exponential function does notnecessarily cross over the two signals representing the referencealtitude and the altitude to be measured, i.e. an exponential functionwhich starts, as an initial value, from any one of the two signalsrepresenting the altitudes can be used. In such a case, a pulse which isgenerated at the instant of the start is used and may be regardedhereunder as a pulse generated at the time of coincidence of theexponential function and the signal representing the altitude from whichthe exponential function starts. This enables to eliminate one of thecomparators 4 or 5 of FIG. 2 which would operate first in case ofcrossing over arrangement by utilizing starting pulse for exponentialvoltage generator 13 in place of the first coincident pulse to controlthe counter 6. Although the description hereafter is based on a decayingexponential function, this invention is not limited to such adecaying-type exponential function.

When the reference altitude H is sea-level and the atmospheric pressureat sea-level is P,, the following equation is obtained from the equation(7 H, l840()(log,,,P, log,,,P,)( l+0.003670) 10 Where altitude isdenoted by 11,, for the mean temperature 0 0; then,

1 gl0 l glO I) l) Therefore, the equation I0) is represented by,

H,=(l +0.003670)H,, 12

Now, a temperature correction of this invention for the barometricaltimeter according to the standard atmosphere adopted by ICAO in 1952is explained. In the ICAO standard atmosphere, the temperature atsealevel is I5C, and it decreases 6.5C for every 1,000m increment ofheight, and becomes 56.5C at the altitude of I I,000m; this temperatureis unchanged until the altitude reaches 20,000m as indicated in FIG. 4.Therefore, if the altitude H, is in the range H, s 11,000m, the meantemperature 0 is,

The value ofk in the equation (9) is denoted by It for the nth layer.Then,

ll ll A!" II II H k At The mean temperature of a thin atmospheric layercan be considered to be the temperature of this layer; therefore, thetemperature of the nth layer is given as follows,

Therefore,

11 =7 |:1+0.00367(150.00(i5 2 mm] i=1 2 ,iT Alof the equation (l9)indicates the altitude of the nth layer. Therefore, if the countingfrequency of the counter 6 shown in FIG. 2 is set to 843! at the time r=t and after that the counter 6 counts the changing frequency k,,according to the altitude as indicated in the equation (19). thealtitude corrected according to the temperature distribution of the [CA0standard atmosphere, is obtained by the counter 6.

FIG. 6 is a block diagram of a temperature correction circuit whichcontrols the counting frequency of the counter 6 according to theequation (I9). In FIG. 6, a voltage-controlled oscillator 21 changes theoscillation frequency in proportion to the output of a summing amplifier31 whose input is connected to signal I and also to the output ofadigital-analog converter 22. The controlling input voltage of thisvoltagecontrolled oscillator 21 is l at z At this time, the oscillationfrequency is 8431 Hz. At Fr that is, when an exponential voltage V iscompared and coincided with the reference voltage V in the comparator 4of FIG. 2, a pulse is applied to a terminal 24 and then a switch 23 isturned ON. Therefore, the output of the voltage-controlled oscillator 21is applied to a counter 26 and the counter 26 begins to count. Anarithmetic circuit 27 is composed ofa fixed memory 29 and an accumulator30. A number 2.261 l l0- is memorized in the fixed memory 29 and isadded in the accumulator 30 through a gate and adder circuit 28acontrolled by an operation command given to a terminal 28 for every onecount of the counter 26. The output of the accumulator 30 is convertedto an analog value by a digitalanalog converter 22 and is applied as oneinput of the summing amplifier 31. An input of l is applied to anotherinput of amplifier 31 and l 2.261 1X10 is obtained at its output whenthe first pulse is applied. Therefore, when the output of the summingamplifier 31 becomes less than I, the oscillation frequency of thevoltage-controlled oscillator 21 becomes slightly lower than 8431 Hz.Thus, the counter 26 counts the frequency of the voltage-controlledoscillator 21 which changes little by little continuously according tothe equation (19), that is, the frequency k A voltagefrequencyconverting circuit of the type, for instance, of a digital voltmeter ofthe voltage-frequency converting type may be adopted as such avoltage-controlled oscillator 21.

Where the altitude is less than 11,000m, the switch 23 is turned OFF byapplying a pulse generated when V =Vto a terminal 25 from the comparator5 of FIG. 2 at the time #1,; thus, the counting of the counter 26 isstopped. In case the altitude is higher than 11,000m, the number 11,000,being memorized in a memory 32, is compared with the contents of thecounter 26 by a comparator 33. When the contents of the counter 26become l L000, a switch 34 is turned OFF in response to an output fromthe comparator 33, and the operation command to the arithmetic circuit27 is no longer applied. At this instant, the content of the accumulator30 is 0.2487 and the oscillation frequency of the volt age-controlledoscillator 21 is 6334 Hz corresponding to the constant temperature 56.5Cat the altitude of lI,OO0m or more. After the time Ft if the counter 26is illustratively of the binary type, the contents of the counter 26indicate the altitude on an indicator 36 through a binary-decimalconverter 35 by command from the control circuit 7 (FIG. 2). Further, areset signal is applied to the terminal 37, and the counter 26 and theaccumulator 30 are reset.

FIG. 7 shows a change of the oscillation frequency of thevoltage-controlled oscillator 21 of FIG. 6. In FIG. 7, the frequency.which is 8431 Hz at the time t t or t=t becomes 6334 H2 corresponding tol 1,000m, 1.4967 seconds later in the case of Fl, and after that thefrequency is constant at this value.

Next, the temperature correction by means of virtually changing thecounting frequency of the counter is explained. In the equation (19),let At, be constant and denoted by Ar and,

Ar,=Ar=l/843l (sec.) 20

then, as the coefficient k,, is,

k,,=84312.26ll l0 Ek, 21)

the thickness of the nth thin layer of the atmosphere measured in thetime interval At becomes the follow ing, as derived from equation l 6),AH,,=l2.261] l0- (l/843l)k, 22

The AH measured in the time interval A! of l/843l seconds at sea-level,is lm, but as the altitude increases, AH becomes less, as indicated inFIG. 5. As (1/8431) k, is the altitude from the level at that point, thetemperature corrected equivalent counting frequency can be executed, forinstance, by counting the cycles of an 8431 Hz signal with the counterand subtracting the value of the counter at that time, multiplied by2.261 1X10 Hz for every one pulse, from the indication of the counter.

FIG. 8 is a circuit block diagram of an illustrative embodiment whichexecutes the temperature correction of this invention. In FIG. 8,numeral 101 identifies a pulse oscillator which oscillates the saidfrequency of 8431 Hz, and numeral 102 represents a switch which turns ONin response to a pulse from the terminal 109 and is turned OFF inresponse to a pulse from a terminal 127. Numeral 103 refers to a T-typeflip-flop and is triggered alternately by a pulse applied through theswitch 102, and its outputs are applied to switches 104 and 105,alternately turning them ON and OFF. Numerals 106, 107 and 108 designatedelay circuits. When a pulse from the comparator 4 of FIG. 2 at the timer=r,,, is applied to the terminal 109, the switch 102 turns ON, and theoutput of the oscillator 101 is counted by a counter 110 through'delaycircuits 106 and 107. The counter 110, an accumulator 102, and asubtractor 113 compose a subtracting circuit 111 for subtracting thecontents of the accumulator 112 from the contents of the counter 110 byan operation command to a terminal 114 through a delay circuit 108; theresults of this operation are fed back into the counter 110. Theaccumulator 112 and a fixed memory 116 compose an accumulating circuit115. In operation, a signal 2.261 1X10, stored in the fixed memory 116,is applied to and summed by the accumulator 112 for each pulse of anoperation command applied by the switch 122 through the terminal 117 togate 117A. The numerals 118, 118A and 1188 denote gates. The gate 118reads the lowest position bit of integer portion, i.e. a 2 position bitof the binary counter 110, and stores this position bit in a binary 1bit memory 119 or 120 through either switch 104 or switch 105, dependentupon which is in its ON state, in response to a pulse applied throughthe delay circuit 106. An inverted digit is given to the memory 120 inthis case.

Now, if the switch 104 is rendered conductive by a pulse from theoscillator 10] and the lowest position bit of integer portion, i.e. the2 position digit of the counter 110, is stored in the memory 119, and ifthe next pulse from the oscillator 10] is applied, the 2 position digitof the counter is inverted and stored in the memory 120. When and onlywhen there is achange in the lowest position digit of the integerportion, i.e. the 2 position of the counter 110 at the times of thesetwo pulses, the contents of the memory 119 and become equal and a pulseis generated by a comparator 121. This pulse serves as an operationcommand to the terminal 117 through the switch 122 when it is in its ONstate. Then, the content of the fixed memory 116 is added once to theaccumulator 112. As an increment of the contents of the counter 110 isalways less than 1 in response to one pulse from the oscillator 101, theaccumulation in the accumulator 112 is surely executed for everyincrement of 1 in the integer portion of the counter 110.

The above process is executed for every 1 pulse of the oscillator 101,i.e. for each l/843l seconds, and although the frequency of theoscillator 101 does not change, the equivalent counting frequency of thecounter 110 changes with time, as indicated by the equation (22). As acoincident pulse from the comparator 5 in FIG. 2 is applied to theterminal 127 at the time t and the switch 102 is turned OFF, the counter110 serves to indicate the altitude at that time. Further, the contentsof the counter 110 are converted by a binary-decimal converter 123 by acommand from the control circuit 7 of FIG. 2, and the altitude isindicated by an indicator 124. Finally, the counter 110 and theaccumulator 112 are reset by a signal applied to the reset terminal.

If the altitude is higher than 11,000m, the integer portion of thecounter 110 is compared with 11,000, i.e. the contents ofa memory 125,in a comparator 126. The switch 122 is turned OFF by a signal derivedfrom the comparator 126, and the accumulator 112 continues to hold theconstant correcting value for the altitude of l 1,000m or more. As aresult, the counter 110 counts the equivalent frequency of 6334 Hz untilthe time Next, an approximate correction for the temperature isexplained below. Using the value of 0 of the equation (13), the term ofthe temperature correction in the equation 12) is given by,

is obtained, but this equation has a small error because H is usedinstead of H in the equation (23). To make this error less, forinstance, at the vicinity of H 10,000m, the next equation is preferable.

An approximation by the equation (25) is described as follows, butsimilar processes are held in the cases of employing other approximatecoefficients.

If c I in equation (6) and 6 in equation (9), then,

= I .06k,,( 0.0000106 (1.06k (t t (27) is obtained.

FIG. 9 shows an illustrative embodiment of a circuit which executes thetemperature correction according to equation (27). In FIG. 9, numeral151 designates a reversible counter which counts up' in response to apulse applied to a terminal 152 and counts down in response to a pulseapplied to a terminal 153. A terminal 154 is a reset terminal and isconnected with a terminal 174. Numeral 155 identifies an oscillatorwhich generates pulses of the frequency of the first term of theequation (27) as follows,

roe/ 847011, (28) Numerals 156, 157 and 158 identify switches and areturned ON in response to pulses to the terminals marked with trianglesand are turned OFF in response to pulses applied to the terminals markedwith circles. The coincident pulse at the time t derived from thecomparator 4 in FIG. 2 is applied through a terminal 159 to switch 156to thereby turn the switch 156 ON. The coincident pulse at the time r,from the comparator of FIG. 2, is applied through a terminal 160 toswitch 156 to turn the switch 156 OFF. Accumulation circuits 161 and 162are composed respectively of a fixed memory 165 and an accumulator 166,and the accumulator 166 and an accumulator 167. In the accumulationcircuit 161, a gate 163A is controlled by an operation command appliedto a terminal 163 and a numerical value 2 X 0.0000106 stored in thefixed memory 165, is accumulated in the accumulator 166. In theaccumulation circuit 162, a gate 164A is controlled by an operationcommand applied to a terminal 164 through a delay circuit 168, and thecontents of the accumulator 166 are non-destructively read out andaccumulated in the accumulator 167.

When the switch 156 has been turned ON at the time r and n pulses havepassed; the content n is stored in the counter 151; the content 2 X0.0000106 X n is stored in the accumulator 166, and the content, 2 X0.0000106 x in 0.0000I06(n +11 is stored in the accumulator 167. Whenthe switch 156 has been turned OFF at the time I the content 1.06k (rr,,) is stored in the counter 151, and the content,

is stored in the accumulator 167.

At this time, the switch 157 is turned ON and a counter 169 begins tocount pulses of the oscillator 155. When the contents of the counter 169have become equal with the contents of the accumulator 167, asdetermined by a comparator 170, the comparator 170 generatesan output toturn the switch 157 OFF. The pulses which have passed through the switch157 in this interval are also applied to the terminal 153 of the counter151. Therefore, when the switch 157 has been turned OFF by a pulseoutput from the comparator 170, the content in the counter 151 is,l.06lt r t.,) 0.0000I06( l.06k (t -t 30 Thus, the altitude H for whichthe temperature correction has been executed according to the equation(27), is present in the above counter 151. The counter output is appliedto an indicator and the altitude H is indicated.

Next, the temperature correction is explained in cases where altitude His higher than 11,000m. At the [CA0 standardatmosphere, the value ofI.06H,,, corresponding to H Il,000m, is 12,697m. Therefore, 12,697 isstored in a fixed memory 171 and, when this stored value coincides withthe contents of the counter 151, the switches 156 and 158 arerespectively turned OFF and ON by an output of a comparator 172, andpulses from an oscillator 173 are applied to and counted by the counter151. By setting the frequency of the oscillator 173 to 6,334 Hzcorresponding to the temperature of the altitude l 1,000m to 20,000m inthe standard atmosphere, the counter 151 continues the countingcorresponding to the altitudes which are higher than 11,000m. Thiscounting is stopped by a pulse applied to a terminal 160 at the time t,.Thereafter, the correction for the temperature distribution of theatmosphere at the altitude lower than I 1,000m is executed in the samemanner as the abovementioned case. During the process of this correctionthe contents of the counter decreases and might become again to I2,697which operates the comparator 172 to turn on the switch 158. To preventthis, the switch 158 is made of the type such that once it turns OFF,then it is kept OFF until the time when processes of this correction arefinished. A reset pulse is applied to the terminal 174 from the controlcircuit 7 of FIG. 2 at the time when processes of the correction havefinished to reset the counters 151, 169, and the accumulators166 and167.

The above correction is an approximate one, but the accuracy for theICAO standard atmosphere is good, and the errors are respectively in theorder of 2m, 16m, 27m, 35m, 9m, and Im at the altitudes of 1,000m,3,000m, 5,000m, 8,000m, 10,000m and 11,000m. There is no additionalerror at the altitude higher than I 1,000m.

The counters 26 of FIG. 6, of FIG. 8, and 151 and 169 of FIG. 9 are theconventional counters whose contents increase by I for 1 input pulse.Alternatively, they may be composed of registers with adders constructedto increase their contents by the specific number a for I input pulse.In this case, the frequency of the oscillator which provides commandpulses, the contents of the memory which is used for the temperaturecorrection, and the operation control circuit have to be modifiedcorrespondingly.

FIG. 10 shows an illustrative embodiment of the modifications of thecounter 110, the temperature correcting circuit. and its control circuitof FIG. 8. In FIG. 10, numeral 201 refers to a terminal corresponding tothe input terminal .of counter 110, numeral 202 designates a registercorresponding to the counter 110, numeral 204 represents an accumulatorcorresponding to the accumulator 112 of FIG. 8, numeral 205 identifies amemory corresponding to the memory 116 of FIG. 8, and numeral 203 refersto a memory. Numeral 206, indicated by dotted lines, designates anadding circuit which adds the content a of the memory 203 to theregister 202 in response to a command pulse applied to the terminal 209,where a is a positive value. Numeral 207, indicated by chain-lines,refers to an adding circuit which adds algebraically the contents of theaccumulator 204 to the register 202 in response to a command pulseapplied to the terminal 210. Numeral 208, indicated by dotted lines,identifies an adding circuit which adds algebraically the content b ofthe memory 205 to the accumulator 204 in response to a command pulseapplied to the terminal 211. Numeral 212 designates an operation controlcircuit which applies to the terminal 211 command pulse(s) equal innumber to the increment of the integer portion of the content of theregister 202 for every pulse applied to the terminal 201.

Provided that the content of the register 202 is X, N pulses are appliedto the terminal 201, and X=O at N=; the content of the accumulator 204becomes 12X. Therefore, when AN more pulses are applied, the incrementof X is given as follows,

AX=aAN+bXAN (31) Therefore, by integrating equation (31),

is obtained. While, from the equations l 7) and l 9),

is obtained. From the equations l 6) and (33),

is introduced. Integrating this equation (34),

Z 2561 (11) (35) is obtained.

Comparing equations (32) and (35), X of the equation (32), i.e. thevalue which is obtained by the content of the register 202, indicatesthe altitude H by replacing as follows,

b/a=2.26ll X 10* 37) Therefore, the altitude H may be obtained bychoosing properly the content a of the memory 203, the content b of thememory 205, and the pulse frequency N/I applied to the terminal 201, tosatisfy the equations (36) and (37) using the circuits indicated in FIG.10, instead of the counter, its correcting circuit, and control in FIG.8.

As mentioned above, the circuits shown in FIG. 10 may be regarded as anequivalent counter which counts a for 1 input pulse in comparison withthe counter 110 in FIG. 8 which counts 1 for each input pulse. In thiscase, a may not be an integer. As a pulse is applied to the terminal 117through the switch 122 in FIG. 8, is generated for every change of the 2position digit of the counter 110, it results to generate command pulsesto the terminal 117 equal in number to the increment of the integerportion of the content of the counter for each input pulse. This processis equivalent in effect to that of the control circuit 212 of FIG. 10.Changing the correction mode at the altitude l 1,000m is also performedsimilarly by generating a control pulse when the content of the register202 has reached a value corresponding the altitude l 1 ,000m to stop theaccumulating operation of the content of the memory 205 to theaccumulator 204.

FIG. 10 can be modified to get the same result as follows. Instead ofstoring number a in the memory 203, it is stored in the accumulator 204,and the content of the memory 205 which represents the absolute value ofb is subtracted from the content of accumulator 204 number of timecorresponding to the increment of the content of register 202, for eachpulse applied at the terminal 201, and the content of the accumulator204 is added to the register 202 corresponding to each pulse applied atthe terminal 201. In this modification the memory 203 can be eliminated.

As stated before, the characteristics of the counter 110, its correctingcircuit, and control circuit of FIG. 8, and the circuits shown in FIG.10 are, briefly, as follows; 1

l. the counter or the register changes its content by a specificnumerical value for every input pulse;

2. the control circuit detects a change of the content of the counter orthe register for each input pulse and generates command pulse(s)corresponding in number to the increment of integer portion ofthecontent;

3. the accumulator accumulates the content of the memory in response tocommand pulse(s); and

4. the content of this accumulator is further accumulated in cascade inthe counter or the register for every input pulse.

The counter 151 of FIG. 9 operates in a manner similar to that describedabove, and can also be constructed to perform the similar action evenwhen a counter is not one which counts 1 for each 1 pulse.

In the embodiment shown in FIG. 9, the circuit operates to accumulateand store the correcting value proportional to the square of thealtitude in the accumulator 167 until the altitude to be measured or Il,000m is attained and then, to subtract this correcting value from itscontents after disabling the counting of the counter 151. The sameresult is also obtained by using a register instead ofthe counter 151,by making I added for each pulse, and by subtracting the contentsaccumulated in the accumulator 166 from the register. In this case, theaccumulator 167, the counter 169, and the comparator 170 can be omitted.Further, the same result is also attained by changing the state of theswitches 156 and 158 in response to the signal from the comparator 172when the content of the register 151 totals I 1,000 and after storingll,000 in the memory 171. The following process is also used to performthe same result. Namely, the register 151 continues the addition of 1for each pulse of the oscillation frequency 8470112 of the oscillatorand the accumulation in the accumulator 166 from the memory is stoppedat the time when the content of the register 151 reaches the value 11,000, and the constant value accumulated in the accumulator N56 issubtracted from the register 151 for every 1 pulse from the oscillator155.

Above descriptions are made for c=l in equation (2), but this inventionis not limited to this specific value of c. For other values of c, it isobvious only to change the numerical values used in the descriptionaccording to the value ofc.

As mentioned above, the barometric altimeter of this invention canmeasure and indicate the altitude by its digital processes and executethe temperature correction automatically in short time. Therefore, itwill prove the excellent performance for aircraft.

Numerous changes may be made in the abovedescribed apparatus and thedifferent embodiments of the invention may be made without departingfrom the spirit thereof; therefore, it is intended that all mattercontained in the foregoing description and in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus to obtain an accurate indication of altitude, saidapparatus comprising:

a. first means for generating a first electrical signal proportional tothe atmospheric pressure at a reference altitude;

b. second means responsive to the atmospheric pressure for generating asecond electrical signal proportional to the atmospheric pressure at thealtitude to be measured;

c. time signal means for generating a signal esponentially varying withtime;

d. coincidence means for generating first and second coincidence signalsrespectively at times t and t when the exponentially varying signalcoincides with the first and second signals, respectively;

e. count means responsive to the first and second coincidence signalsfor counting at a selected counting frequency during the intervalbetween the first and second coincidence signals; and

f. correction means for correcting the contents of said count means withregard to the temperature of the atmosphere.

2. Apparatus as claimed in claim 1, wherein said correcting means variesthe selected counting frequency of said count means according to thetemperature distribution.

3. Apparatus as claimed in claim 1, wherein said correction means variescontinuously with time the selected counting frequency of said countmeans according to the temperature distribution.

4. Apparatus as claimed in claim 1, wherein said count means includesoscillator means for generating and applying the selected countingfrequency to said count means, and further including accumulator meansfor receiving and storing an incremental input for every count-pulse ofsaid count means, for controlling said selected counting frequency inaccordance with the content of said accumulator means.

5. Apparatus as claimed in claim 4, wherein said oscillator means isresponsive to a predetermined count of said count means corresponding toa predetermined altitude, for generating subsequently the constantfrequency.

6. Apparatus as claimed in claim 1, wherein said correction meanscomprises arithmetic means coupled to said count means for subtractingthe number determined by the counted contents of said count means, fromthe accumulated contents of said count means, to correct for temperaturedistribution.

7. Apparatus as claimed in claim 6, wherein there is further includedsecond arithmetic circuit comprising accumulator means for storing apredetermined number therein, said accumulator means responsive to theincremental value of said count means for accumulating the number storedin said memory means, to determine thereby the number determined by thecounted contents of said count means.

8. Apparatus as claimed in claim 7, wherein there is further includeddetection means associated with said count means for detecting a changeof the 2 digit of said count means, and said second arithmetic meanscomprising a memory for storing a predetermined number therein, saidsecond arithmetic means responsive to each change of the 2 digit forapplying the number in said memory means to said accumulator means foraccumulation therein.

9. Apparatus as claimed in claim 6, wherein said arithmetic means isoperative in a first mode for determining altitudes less than a specificaltitude and in a second mode for determining altitudes greater than thespecific altitude, said arithmetic circuit responsive to said countmeans when it has counted a number corresponding to the specificaltitude to be disposed from its first to its second mode of operation.

10. Apparatus as claimed in claim 1, wherein said count means isresponsive at time r,, to the occurrence of the first coincident signalto initiate its counting operation, and responsive at time t, to thesecond coincident signal to terminate its counting operation.

11. Apparatus as claimed in claim 10, wherein said correction means isresponsive to the counted contents of said count means for compensatingfor temperature in a manner proportional to the square of the countedcontents of said count means after said count means has terminated itscounting operation in response to said second coincidence signal.

12. Apparatus as claimed in claim 11, wherein said correction meanscomprises first and second accumulator means and memory means forstoring a predetermined number therein, said correction means responsiveto each pulse counted by said count means for accumulating the storednumber in said first accumulator means and for accumulating in cascadethe contents of said first accumulator means in said second accumulatormeans, to obtain a compensation value proportional to the square of thecounted contents of said count means.

13. Apparatus as claimed in claim 1, wherein said correction meansincludes first oscillator means for generating a first clock signal of afirst frequency and second oscillator means for generating a secondclock signal of a second, different frequency from that of the firstfrequency, said correction means operative in a first mode for applyingthe first clock signal to said count means until said count means countsa selected number of clock pulses corresponding to a specific altitude,and in a second mode after said count means has counted the selectednumber of clock pulses for applying the second clock signal to saidcount means, said correction means subtracting a value proportional tothe square of the number of pulses counted by said count means inresponse to the first clock signal after said count means has stoppedcounting.

14. Apparatus as claimed in claim 1, wherein said correction meanscomprises subtraction means, said count means being responsive to aseries of clock pulses for counting the clock pulses, and saidsubtraction means subtracts the number proportional to the countedcontents of said count means from each count of said count means.

15. Apparatus as claimed in claim 1, wherein there is further includeddisplay means responsive to said count means for providing an indicationof the contents of said count means after said count means has stoppedcounting and said correction finished.

16. A barometric altimeter comprising:

a. first means for generating a first signal proportional to theatmospheric pressure at a reference altitude;

b. second means for generating a second signal proportional to theatmospheric pressure at the altitude to be measured;

0. third means for generating a signal varying exponentially with time;

d. coincidence means for generating first and second coincidence signalsat times and I, when the signal of said third means coincides with thefirst and second signals, respectively;

e. clock pulse generating means for generating clock pulses andresponsive to the first coincidence signal to initiate its operation andresponsive to the second coincidence signal to terminate its operation;

f. memory means for storing a predetermined number therein;

g. accumulator 'means to accumulate the predetermined number of the saidmemory means therein;

h. register means responsive to each clock pulse for varying the contentof said register means in accordance with the content of saidaccumulator means during the interval between the times t and 1;

i. command pulse generating means for accumulating predetermined numberof said memory means in said accumulator means a number of times inaccordance with the increment of the content of said register means foreach clock pulse.

17. A barometric altimeter as claimed in claim 16, wherein there isfurther included means to discontinue the accumulation of the content ofsaid memory means to said accumulator means when the content of the saidregister means has reached a predetermined value.

18. A barometric altimeter as claimed in claim 16, wherein there isfurther included display means for providing an indication of thecontents of said register after the termination of the generating of theclock pulses.

1. Apparatus to obtain an accurate indication of altitude, saidapparatus comprising: a. first means for generating a first electricalsignal proportional to the atmospheric pressure at a reference altitude;b. second means responsive to the atmospheric pressure for generating asecond electrical signal proportional to the atmospheric pressure at thealtitude to be measured; c. time signal means for generating a signalesponentially varying with time; d. coincidence means for generatingfirst and second coincidence signals respectively at times to and t1when the exponentially varying signal coincides with the first andsecond signals, respectively; e. count means responsive to the first andsecond coincidence signals for counting at a selected counting frequencyduring the interval between the first and second coincidence signals;and f. correction means for correcting the contents of said count meanswith regard to the temperature of the atmosphere.
 2. Apparatus asclaimed in claim 1, wherein said correcting means varies the selectedcounting frequency of said count means according to the temperaturedistribution.
 3. Apparatus as claimed in claim 1, wherein saidcorrection means varies continuously with time the selected countingfrequency of said count means according to the temperature distribution.4. Apparatus as claimed in claim 1, wherein said count means includesoscillator means for generating and applying the selected countingfrequency to said count means, and further including accumulator meansfor receiving and storing an incremental input for every count-pulse ofsaid count means, for controlling said selected counting frequency inaccordance with the content of said accumulator means.
 5. Apparatus asclaimed in claim 4, wherein said oscillator means is responsive to apredetermined count of said count means corresponding to a predeterminedaltitude, for generating subsequently the constant frequency. 6.Apparatus as claimed in claim 1, wherein said correction means comprisesarithmetic means coupled to said count means for subtracting the numberdetermined by the counted contents of said count means, from theaccumulated contents of said count means, to correct for temperaturedistribution.
 7. Apparatus as claimed in claim 6, wherein there isfurther included second arithmetic circuit comprising accumulator meansfor storing a predetermined number therein, said accumulator meansresponsive to the incremental value of said count means for accumulatingthe number stored in said memory means, to determine thereby the numberdetermined by the counted contents of said count means.
 8. Apparatus asclaimed in claim 7, wherein there is further included detection meansassociated with said count means for detecting a change of the 2* digitof said count means, and said second arithmetic means comprising amemory for storing a predetermined number therein, said secondarithmetic means responsive to each change of the 2* digit for applyingthe number in said memory means to said accumulator means foraccumulation therein.
 9. Apparatus as claimed in claim 6, wherein saidarithmetic means is operative in a first mode for determining altitudesless than a specific altitude and in a second mode for determiningaltitudes greater than the specific altitude, said arithmetic circuitresponsive to said count means when it has counted a numbercorresponding to the specific altitude to be disposed from its first toits second mode of operation.
 10. Apparatus as claimed in claim 1,wherein said count means is responsive at time to to the occurrence ofthe first coincident signal to initiate its counting operation, andresponsive at time t1 to the second coincident signal to terminate itscounting operation.
 11. Apparatus as claimed in claim 10, wherein saidcorrection means is responsive to the counted contents of said countmeans for compensating for temperature in a manner proportional to thesquare of the counted contents of said count means after said countmeans has terminated its counting operation in response to said secondcoincidence signal.
 12. Apparatus as claimed in claim 11, wherein saidcorrection means comprises first and second accumulator means and memorymeans for storing a predetermined number therein, said correction meansresponsive to each pulse counted by said count means for accumulatingthe stored number in said first accumulator means and for accumulatingin cascade the contents of said first accumulator means in said secondaccumulator means, to obtain a compensation value proportional to thesquare of the counted contents of said count means.
 13. Apparatus asclaimed in claim 1, wherein said correction means includes firstoscillator means for generating a first clock signal of a firstfrequency and second oscillator means for generating a second clocksignal of a second, different frequency from that of the firstfrequency, said correction means operative in a first mode for applyingthe first clock signal to said count means until said count means countsa selected number of clock pulses corresponding to a specific altitude,and in a second mode after said count means has counted the selectednumber of clock pulses for applying the second clock signal to saidcount means, said correction means subtracting a value proportional tothe square of the number of pulses counted by said count means inresponse to the first clock signal after said count means has stoppedcounting.
 14. Apparatus as claimed in claim 1, wherein said correctionmeans comprises subtraction means, said count means being responsive toa series of clock pulses for counting the clock pulses, and saidsubtraction means subtracts the number proportional to the countedcontents of said count means from each count of said count means. 15.Apparatus as claimed in claim 1, wherein there is further includeddisplay means responsive to said count means for providing an indicationof the contents of said count means after said count means has stoppedcounting and said correction finished.
 16. A barometric altimetercomprising: a. first means for generating a first signal proportional tothe atmospheric pressure at a reference altitude; b. second means forgenerating a second signal proportional to the atmospheric pressure atthe altitude to be measured; c. third means for generating a signalvarying exponentially with time; d. coincidence means for generatingfirst and second coincidence signals at times t0 and t1 when the signalof said third means coincides with the first and second signals,respectively; e. clock pulse generating means for generating clockpulses and responsive to the first coincidence signal to initiate itsoperation and responsive to the second coincidence signal to terminateits operation; f. memory means for storing a predetermined numbertherein; g. accumulator means to accumulate the predetermined number ofthe said memory means therein; h. register means responsive to eachclock pulse for varying the content of said register means in accordancewith the content of said accumulator means during the interval betweenthe times t0 and t1; i. command pulse generating means for accumulatingpredetermined number of said memory means in said accumulator means anumber of times in accordance with the increment of the content of saidregister means for each clock pulse.
 17. A barometric altimeter asclaimed in claim 16, wherein there is further included means todiscontinue the accumulation of the content of said memory means to saidaccumulator means when the content of the said register means hasreached a predetermined value.
 18. A barometric altimeter as claimed inclaim 16, wherein there is further included display means for providingan indication of the contents of said register after the termination ofthe generating of the clock pulses.